Filter media specification for rapid gravity filters The recommended uniformity coefficient (U.C.) for good quality filter sand used in rapid gravity filters should be approximately:

Introduction / Context:
In rapid gravity filtration, the filter media gradation critically affects hydraulic performance, head loss, and particle capture. The uniformity coefficient (U.C.) indicates how uniformly graded the sand is and is defined as D60/D10, where D60 and D10 are particle sizes at 60% and 10% finer by weight.

Given Data / Assumptions:

• Filter media is silica sand used in rapid gravity filters.
• Goal is a practical U.C. that balances run length and backwash behaviour.

Concept / Approach:
A lower U.C. indicates a more uniform gradation. Standard practice for rapid gravity sand filters targets effective size (D10) around 0.45–0.70 mm and a U.C. not exceeding about 1.7–2.0. This ensures predictable hydraulic gradients and efficient backwashing without excessive media loss.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals consistently indicate U.C. less than about 1.7–2.0 for rapid filters; higher values imply broad gradation, reducing performance.

Why Other Options Are Wrong:

• 3, 4, 5: too broadly graded; lead to non-uniform flow paths and poorer performance.
• None of these: incorrect, because U.C. ≈ 2 is a recognized practical limit.

Common Pitfalls:
Confusing slow sand filter specs (finer media) with rapid filters and tolerating overly broad gradation.

Final Answer:
2